1. Field of the Invention
The present invention relates to a helical scan type (VTR) magnetic recording and reproducing apparatus constructed to enable high quality playback of a still image.
2. Description of the Prior Art
As is well known, a helical scan type VTR records and reproduces each field or frame of video signal sequentially by two rotary heads following a record locus which is oblique to the length of a magnetic tape. It is constructed to record a vertical blanking signal at an end of each record locus such that a transition point from one record track to the next track, that is a switching point of the rotary heads, lies near the vertical blanking signal.
When the magnetic tape is stopped, a still image can be reproduced. However, the scanning locus of the rotary head on the magnetic tape when the magnetic tape is stopped does not coincide with the record locus. Accordingly, the rotary head cannot exactly scan the record locus, and there exists a time period when no play back signal is reproduced.
If this zero-playback signal period lies near the vertical blanking signal, an image including a substantially small defect can be displayed on the television receiver. Accordingly, prior art apparatus has stopped the movement of the magnetic tape slowly during the time that the zero-playback signal period lies near the vertical blanking signal, when a still image is to be reproduced, as shown in the U.S. Pat. No. 4,280,146 to Misaki et al.
This situation is explained in detail in FIGS. 1, 2 and 3(A)-3(D). In FIG. 1, element 1 is a rotary driven head cylinder which rotates in the direction of the arrow; two magnetic heads, A.sub.1, and B.sub.1, having the head gaps which are inclined at angles which differ by a few degrees are fitted on the head cylinder 1.
A magnetic tape 2 moves in the direction of the arrow Y, and is wound helically around a 180.degree. circumference, and is guided by the guide parts 3 and 4 along the circumference of the head cylinder 1.
FIG. 2 shows a typical recording pattern on the magnetic tape 2.
The recording locus a.sub.1, a.sub.2 . . . and the recording locus b.sub.1, b.sub.2 . . . are azimuth recorded by the rotary head A.sub.1 and the rotary head B.sub.1 respectively.
FIGS. 3(A)-3(B), show typical respective scanning loci T for the rotary magnetic heads A and B during the playback of the still image.
Since the rotary magnetic heads A.sub.1 and B.sub.1 reproduce only their own recording locus, a.sub.1, a.sub.2. . . and b.sub.1, b.sub.2 . . . respectively, during the playback, the playback signal of FIG. 3(A), as shown in FIG. 3(C), shows the playback signal of the magnetic head A.sub.1 from the recording locus a.sub.1 and a.sub.2 during the first field duration F.sub.1, and during the subsequent field duration B.sub.2, the playback signal from the recording locus b.sub.1 of the magnetic head B.sub.1 is shown.
As clearly seen in FIG. 3(C), in the middle of the field duration F.sub.1, during the transfer of the recording locus from a.sub.2 to a.sub.1, the play back signal is not obtained sufficiently, resulting in a deterioration of the still image.
Whereas, the playback signal of the configuration of FIG. 3(B), as shown in FIG. 3(D), shows the playback signal from the recording locus a.sub.1 during the first field duration F.sub.1 by the magnetic head A.sub.1, and during the subsequent field duration F.sub.2, the playback signal from the recording locus b.sub.1 of the magnetic head B.sub.1 is shown.
Since the duration of the minimum level of the playback signal exists in the beginning or at the end portion in each field (adjacent portion of the vertical blanking duration of the playback signal), a better still image is obtainable in contrast to the case in FIG. 3(C).
However, this still picture is not sufficient.
There are two defects.
One defect is that the still picture contains noisy portions at the beginning and end of the playback image.
Another defect is that this still picture moves slightly due to the fact that the still picture is composed of two field pictures of record locus (a.sub.1, b.sub.1) reproduced by two heads (A.sub.1, B.sub.1) respectively. This reproducing of this still picture is called a reproducing of a "Frame Still Picture".